The present invention relates to low level light detecting devices and, more particularly, to a device capable of detecting receipt of individual photons over a wavelength range extending into the infrared region.
It is desirable in a number of light detecting applications to be able to sense the receipt of individual photons, since this represents the upper limit for resolution and sensitivity. Conventional prior art photon detectors have typically utilized photomultiplier tubes. When an incoming photon strikes the photocathode of a photomultiplier tube, an electron of the cathode material is given sufficient energy to escape from the surface of the cathode. The electron travels through the vacuum of the tube and enters an electron multiplier where it strikes a series of successively more positive electrodes. At each impact of the electron, additional secondary electrons are released, and further multiplication occurs. The movement of these electrons through the photomultiplier tube constitutes an electric current flow which may be detected, providing an indication that at least one photon has been received by the tube and has struck the surface of the photocathode.
Photomultipliers, however, have a number of disadvantages. Typically, they are relatively expensive, bulky and require substantial biasing voltages. Additionally, they are appropriate for use in a somewhat limited spectral range. Specifically, infrared photons of relatively long wavelength (greater than one micron) typically do not carry enough energy to release an electron from the cathode surface into the tube vacuum.
For a number of light sensing applications where the detection of individual photons is not required, semiconductor photodetectors, such as photodiodes, are utilized. Photodiodes are small, rugged, relatively inexpensive, and have a broader spectral response than photomultiplier tubes.
Photodiodes, however, as used in prior art devices, are not sufficiently sensitive to be able to detect individual photons. In a typical prior art circuit, a photodiode is reverse biased. A number of thermally generated free charge carriers are available in the diode material and provide conduction of a reverse or "dark" current through the device. When the diode is illuminated, photons strike the diode and impart sufficient energy to elevate additional electrons from the valence band into the conduction band and cause an additional light-generated "photo-current" to flow. This photocurrent is taken to indicate the absorption of light by the diode. However, only a single charge carrier pair is ordinarily generated by the absorption of an optical or infrared photon, and the resulting current is insufficient by itself to allow detection of an individual photon.
It is known to increase the reverse bias of such a photodiode until free charge carriers moving in the diode can pick up enough energy from the internal electric field to release additional carriers in collisions with the diode crystal lattice. These carriers release additional carriers in a process known as avalanche multiplication. Avalanche multiplication multiplies both dark current and photocurrent, with the result that a much larger current proportional to the original unmultiplied current flows in the diode. A diode operated in this mode, especially one specifically intended for such operation, is called an "avalanche photodiode."
The factor by which the current is multiplied by the avalanche effect is very critically dependent upon the bias voltage applied, and this dependence becomes rapidly more critical as the bias is raised to increase avalanche gain. If the bias is raised high enough, the multiplication factor becomes effectively infinite, a condition called avalanche breakdown. If an avalanche breakdown condition were permitted to persist, the avalanche diode would be quickly damaged. In order to control avalanche gain and avoid the dangers of avalanche breakdown, avalanche photodiodes are commonly operated with constant current bias. A set current is supplied to the diode, and the bias voltage is allowed to rise, increasing the avalanche gain until the gain is high enough so that the sum of dark current and photocurrent, multiplied by the gain, is equal to the bias current supplied. If the photocurrent increases, the required gain and therefore the required voltage bias decrease, thereby indicating the absorption of light. An alternative biasing arrangement, carefully controlled fixed voltage biasing, has proven less successful in the past due to the very critical bias control requirements and inherent instability of such a mode of operation.
The fact that a substantially steady dark current is required in order to help set the bias point of a continuously operating avalanche photodiode prevents its use as a detector of individual photons. The thermal generation rate of carriers in the diode must be high enough to provide a substantially steady current, and the fluctuations in that rate will mask the effects of a single charge carrier pair generated by an individual photon. Accordingly, this conventional mode of operation cannot be used to detect individual photons. Photodiodes, however, are well suited for use in infrared detection, since the energy carried by an infrared photon is sufficient to create a charge carrier pair in a suitable semiconductor.
It is known to cool a photosensitive semiconductor device to reduce the noise produced by thermally created free charge carriers in the device, as shown by U.S. Pat. Nos. 3,457,409 issued July 22, 1969, to Shenker et al; 3,445,659, issued May 20, 1969, to Guimento et al; 4,134,447, issued Jan. 16, 1979, to Frosch et al; 3,114,041, issued Dec. 10, 1963, to Amsterdam; 3,103,585, issued Sept. 10, 1963, to Johnson et al; 3,597,614, issued Aug. 3, 1971, to Bishop; 3,602,714, issued Aug. 31, 1971, to Farmer et al; 3,942,010, issued Mar. 2, 1976, to Peterson et al; 4,059,764, issued Nov. 22, 1977, to Belasco et al; and 4,118,947, issued Oct. 10, 1978, to Diedrich et al. While improving the sensitivity of the semiconductor photodetectors, these photodetectors are, nevertheless, not sufficiently sensitive so as to be able to detect individual photons. As noted previously, an avalanche photodiode, biased in a constant current mode, cannot take full advantage of the reduction in dark current produced by cooling.
Accordingly, it is seen that there is a need for a photosensitive semiconductor detector capable of detecting individual infrared photons.